Mammals in general are excellent navigators. They go efficiently from place to place in known environments and are quick to learn their way around new regions. The thrust of our research is to investigate how the complex spatial problem solving abilities of mammals arise from the properties of their nervous systems. We have chosen to study this problem in rats for several reasons. First, a great deal is known about navigation by rats; their capacities in this regard are understood better than those of any other species save man. Second, rats are small enough that interesting environments for observing them during navigation can be created economically. Finally, the relationship between brain function and navigation has been studied much more thoroughly for rats than for any other species. The current view of the neural basis of rat navigation rests in large part on the work of O'Keefe and Nadel (43), who forcefully argued that a major part of the cortex called the "hippocampus" must be intact for rats to show highly intelligent spatial behavior. Just as importantly, they argued that the existence of "place cells" in the hippocampus is prima facie evidence that the hippocampus is directly involved in navigation. Place cells are neurons that have the astonishing property of being intensely active only when a rat is in a restricted portion of its environment -- their activity is "location-specific". The themes of the proposed work have in common a desire to learn more about the meaning of the location-specific activity of place cells. The first concerns the origins of place cell activity; we intend to record from cells in brain regions that provide input to place cells. The second theme involves efforts to more precisely establish the linkages between place cell activity and the choices made by an animal as it solves a spatial problem. The final theme asks about the activity of groups of place cells; we imagine that much can be learned about the nature of the posited hippocampal representation of the animal's environment by finding conditions under which the location- specific activity of individual cells stays in register or shifts. It is our contention that studying the neural basis of navigation is important in the general realm of animal behavior. In a sense, navigation characterizes animal behavior; it is an essential part in the ability of animals to perform all of their self- and species-preserving tasks. It is also hoped that the proposed work will be of value in understanding dementia in people; the relation between navigation and human mental function is so basic that an inability to describe one's location in time and space is taken as direct evidence of severe malfunction during neurological or psychiatric examinations. If animal models are valid for neural processes as complex as getting around in the world, this hope may well be realized.